In the fabrication of printed wiring boards and/or printed circuit boards, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin and non-rosin, or water soluble, fluxes. The rosin fluxes, which are generally non-corrosive and have a much longer history of use, are still widely used throughout the electronics industry. The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. Because they contain strong acids and/or amine hydrohalides and are thus corrosive, the water soluble soldering fluxes can cause circuit failure if residual traces of the material are not carefully removed. For that reason military specifications require the use of rosin fluxes.
Even the use of rosin soldering fluxes, however, can lead to premature circuit failure due to decreased board resistance if traces of residual flux are not removed following soldering. While water soluble fluxes can be easily removed with warm, soapy water, the removal of rosin flux from printed wiring boards has traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane, or mixtures or azeotropes of these solvents. These solvents are undesirable, however, because they are toxic. Thus, their use is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA), and stringent containment equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
Alkaline cleaning compounds known as the alkanol amines, usually in the form of monoethanolamine, have been used for rosin flux removal as an alternative to the toxic chlorinated hydrocarbon solvents. These compounds chemically react with rosin flux to form a rosin soap through the process of saponification. Other organic substances such as surfactants or alcohol derivatives may be added to these alkaline cleaning compounds to facilitate the removal of such rosin soap. Unfortunately, these compounds like the water soluble soldering fluxes have a tendency to cause corrosion on the surfaces and interfaces of printed wiring boards if they are not completely and rapidly removed during the fabrication process.
In another approach, Bakos et al. [U.S. Pat. No. 4,276,186] have used mixtures of N-methyl-2-pyrrolidone and a water miscible alkanolamine to remove solder flux and solder flux residue from integrated circuit modules. These mixtures were also said to be useful for removing various cured synthetic organic polymer compositions such as cured polyimide coating compositions from integrated circuit chip modules.
During the manufacture of printed wiring boards, it is sometimes necessary to temporarily protect certain portions of the board from processing steps such as the process of creating corrosion resistant gold connecting tabs at the board edges. This transient protection of portions of the circuit board can be achieved by the application of special adhesive tape to susceptible areas. Once such protection is no longer needed, the adhesive tape must be removed. A residue of the tape adhesive generally remains which, if not throughly removed, can cause premature board failure. Removal of this tape residue has traditionally been carried out by the use of chlorinated solvents which, as already described, are toxic and environmentally undesirable.
A further procedure that is commonly carried out during printed circuit board fabrication is the application of a soldermask. As the name implies, a soldermask is a polymer or resin coating that is selectively applied to a printed circuit board to shield areas where solder is not required or desired. Where such masking is to be permanent, the soldermask must be "cured," a process by which monomeric reactants are made to polymerize. If curing of the soldermask is incomplete, solder will penetrate areas that are to be shielded, resulting in destruction of the board.
One kind of soldermask is based on acrylate chemistry and is photo-initiated and polymerized, and thus cured, using ultraviolet (U.V.) light. The use of U.V.-cured soldermask is advantageous in that its curing is rapid, but the thickness of such soldermask films must be uniform for successful curing. The U.V. irradiation used cannot penetrate to the bottom of deep soldermask deposits, and attempts to increase the intensity or the duration of irradiation to cure thick sections can result in the embrittlement of the thinner mask film regions.